Pit Bull Mix Case

Species:       CANINE

Breed:          Pit Bull Mix

Age:              8 Y

Gender:        Male Neutered

Source/History:

10 to 15 mm circular hairless mass in right flank. Present 6 to 8 weeks, 5 days ago cytology showed mast cell tumor.

Click here for the IDEXX Pathology Report on this case.

Microscopic Description:

The specimen is characterized by a poorly demarcated and nonencapsulated proliferation of atypical individualized round cells. These cells are proliferating as solid sheets, cords and nests within the deep dermal connective tissues and subcutis. Individual cells are characterized by moderate amphophilic cytoplasm with variable basophilic cytoplasmic granulation and mildly pleomorphic euchromatic nuclei with small nucleoli. There is less than one mitotic figure per 10 high power fields. Eosinophils are present throughout the mass. Excision is complete. The deep margin is 7.3 mm, the lateral margin is 5.3 mm.

Interpretation:

Haired skin: Cutaneous mast cell tumor, grade 2, low

Mitotic Index:

<1 (<1 mitotic figures per 10 high power fields)

Excision:

Complete, deep margin 7.3 mm, narrowest lateral margin 5.3 mm

Vascular Invasion:
Not observed.

Comments:

This mast cell tumor (MCT) is classified as a grade II or intermediate grade tumor based upon the Patnaik grading system (I = low grade, II = intermediate grade, III = high grade). The majority of canine mast cell tumors (up to 80%) are classified as grade II. As such, this is a relatively large subgroup of mast cell tumors, and variable clinical behavior can be anticipated.

Recent work (Kiupel et al. 2011) suggests that cutaneous mast cell tumors can be more usefully graded on a 2-tier system as low grade or high grade. Median survival time for high grade MCTs on this system was less than 4 months, while the median survival time for low grade tumors was not reached after 2 years of follow-up. Using this new grading system, which has been shown to have greater consistency between pathologists than the Patnaik system, this neoplasm is low grade. Studies have suggested multiple tumors do not carry a worse prognosis than single tumors when all tumors are adequately excised. Initial size at the time of surgery is highly predictive of outcome; tumors greater than 3 cm in diameter have decreased survival time. Similarly, decreased survival time is seen with tumors that are incompletely excised; those demonstrating rapid growth, arising from the preputial, subungual, mucocutaneous and muzzle location; or in patients with clinical signs at the time of diagnosis (JAVMA 2006, 228:91-5, Vet Pathol, 48,1:156-68). One retrospective study demonstrated that mitotic index is a valuable prognostic indicator, independent of the grade. For grade II MCTs with a mitotic index of less than 5 or equal to 5, median survival was longer than 70 months, whereas those tumors with a mitotic index of greater than 5 per 10 high power fields had a mean survival of approximately 5 months (Vet Pathol 2007 May, 44. 3:335-41).

Additional prognostic information and therapeutic guidance for grade II MCTs may be obtained by performing additional testing on this sample. Mast Cell Tumor Prognostic Panel 1 (test code 3375) includes immunohistochemical stains for KIT and Ki67, histochemical staining for AgNOR, and PCR analysis for mutations in exon 8 and exon 11 of the cKIT gene. Mast Cell Tumor Prognostic Panel 2 (test code 3373) includes only the staining component (KIT, Ki67, AgNOR). Mast Cell Tumor Prognostic Panel 3 (test code 3374) includes only the PCR analysis for mutations in exon 8 and exon 11 of the cKIT gene.

Discussion with an oncologist regarding prognostic benefits of the mast cell tumor panel is recommended.

Image Comments:

Image 1:  Low magnification view of the entire tumor (as submitted).

Image 2:  Higher magnification view demonstrating the neoplastic mast cells, with few intratumoral eosinophils.

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